As used herein, fullerene is any carbonaceous material wherein the structure is a regular, three dimensional network of fused carbon rings arranged in any one of a number of possible structures including, but not limited to, cylindrical, spherical, ovoid, oblate or oblong. Common fullerenes include the cylindrical carbon nanotube and the icosahedral C60 carbon molecules. In particular, the fullerene is preferably selected from the group consisting of C60, C72, C84, C96, C108, C120, single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT).
Derivatization of planar and other macroscopic silica surfaces is well known and in the public domain. Silica coated fullerenes, including silica coated single walled or multiwalled carbon nanotubes (s-SWNT or s-MWNT) and silica coated C60 molecules, have been made by the process described in US/PCT Application 20,050,089,684 entitled “Coated Fullerenes, Composites and Dielectrics made Therefrom.” Since the discovery of silica coated fullerenes in 2002 researchers have been searching for ways to manipulate them chemically. While there have been many reports and review articles on the production and physical properties of chemically functionalized fullerenes and in particular carbon nanotubes, reports on chemical manipulation of silica coated fullerenes, including silica coated C60 molecules and silica coated carbon nanotubes, have been non-existent.
Single walled carbon nanotubes and multiwalled carbon nanotubes are elongated members of the fullerene family. Since their discovery they have come under intense multidisciplinary study because of their unique physical and chemical properties and their possible applications. Single walled carbon nanotubes can be either metallic or semiconducting, depending on their helicity and diameter. More importantly it has been shown that these properties are sensitive to the surrounding environments. For example, the presence of O2, NH3 and many other molecules can either donate or accept electrons and alter the overall conductivity of the single walled carbon nanotubes. Such properties make single walled carbon nanotubes ideal for nanoscale sensing materials. Nanotube field effect transistors have already been demonstrated as gas sensors. However, to introduce selectivity to nanotube sensors, certain functional groups that can selectively bind to specific target molecules need to be anchored on the nanotube surface. Unfortunately, functionalization changes the electronic properties from semiconductor or conductor to insulating, and at present chemical functionalization is not regiospecific. A further major obstacle to such efforts has been diversity of tube diameters, chiral angles, and aggregation states of the tubes. Aggregation is particularly problematic because the highly polarizable, smooth sided single walled carbon nanotubes readily form bundles or ropes with van der Waals binding energy of ca. 500 eV per micrometer of tube contact. This bundling perturbs the electronic structure of the tubes and precludes the separation of single walled carbon nanotubes by size or type; it also precludes the use of single walled carbon nanotubes as individual tubes in any matrix or solvent.
Individual single walled carbon nanotubes may be obtained encased in a cylindrical micelle, by ultrasonically agitating an aqueous dispersion of raw single walled carbon nanotubes in a suitable surfactant. However, upon drying or attempting to incorporate into other solvents or matrices bundles re-form. Single walled carbon nanotubes have been encased in a wide range of organic materials. It would be desirable to fabricate individually coated single walled carbon nanotubes where the coating is retained in solution and the solid state. Of particular interest are dielectric materials such as silica, which may also be compatible with composite matrix materials. Thick coatings of SiO2 on multi walled carbon nanotubes has been reported, while thin layers have been reported on single walled carbon nanotubes. Experimental measurements and theoretical calculations have shown that the silica-coated nanotubes retain the electronic and optical properties of the uncoated nanotubes. The silica coating does not interfere with the properties of the nanotube. However, while these routes allow for isolation of individual nanotubes the surfaces are defined by the surface chemistry of the silica coating. Oxide and hydroxide generally terminate the surface of silica groups. This is a severe limitation on the application of such materials.